Does battery drain depend on how many people are on a call?
Yes, every person in the call has a video feed that must be decoded and rendered.
So does it take more battery to render several small video feeds versus one large one? If I'm on a call with a bunch of people, does "speaker mode" take less battery than "gallery mode"? Somehow I doubt it would be noticeable, if the effect exists at all.
Possibly, but it would have to be way better specified. You have dimensions like the OS, conference service, camera quality, browser, etc. that all affect it.